Revenue and Contract Balances - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Jun. 29, 2025	Jun. 30, 2024	Jun. 29, 2025	Jun. 30, 2024	Sep. 29, 2024
Revenue from Contract with Customer [Abstract]
Contract liability revenue recognized during the period			$ 213,000	$ 214,000
Net favorable (unfavorable) revenue and operating income adjustments	$ 9,900	$ 16,500	6,500	$ 16,400
Liabilities for anticipated losses	12,800		12,800		$ 15,100
Estimated cost to complete the related contracts	73,000		$ 73,000		$ 101,000
Period for billing and collecting unbilled receivables			12 months
Remaining unsatisfied performance obligation	$ 4,239,772		$ 4,239,772
Remaining performance obligation, termination notice period one			30 days
Remaining performance obligation, termination notice period two			60 days
Remaining performance obligation, termination notice period three			90 days

Represents the amount of remaining estimated costs to complete the contracts.

No definition available.

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Balance Type:	debit
Period Type:	instant

- Definition

Revenue, Remaining Performance Obligation, Termination Notice Period One

+ References

No definition available.

+ Details

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- Definition

Revenue, Remaining Performance Obligation, Termination Notice Period Three

+ References

No definition available.

+ Details

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Namespace Prefix:	ttek_
Data Type:	xbrli:durationItemType
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Period Type:	duration

- Definition

Revenue, Remaining Performance Obligation, Termination Notice Period Two

+ References

No definition available.

+ Details

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Namespace Prefix:	ttek_
Data Type:	xbrli:durationItemType
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- Definition

Represents the period within which substantially all unbilled receivables are expected to be billed and collected.

+ References

No definition available.

+ Details

Name:	ttek_UnbilledContractsReceivablePeriodForBillingAndCollection
Namespace Prefix:	ttek_
Data Type:	xbrli:durationItemType
Balance Type:	na
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- Definition

Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in measure of progress which (increases) decreases obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-10

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-8

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Cumulative provision for contract losses not offset against related costs accumulated on the balance sheet.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 605
-SubTopic 35
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481187/605-35-45-2

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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Balance Type:	na
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- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
-SubTopic 10
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-13

+ Details

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Balance Type:	credit
Period Type:	instant